It is well known that air conditioning systems must be designed to alter both the temperature and the humidity of supply air to achieve specified thermal conditions in a conditioned space. Typically, moisture vapour production within a conditioned space is modest, and so most of the latent thermal energy load on an conditioning system is incurred when reducing the humidity of supply (or fresh) air, to a specified range, rather than removing water vapour from recirculated air flow within the space.
In a building, for example, in addition to sensible heat that must be added or removed to maintain a specified internal building temperature, excess water vapour may also have to be removed from air entering the building to maintain a specified internal humidity range. Such sensible heat and humidity control typically forms part of an air-conditioning and ventilation system.
The typical specified range for ventilated air entering occupied buildings, which may include residential and commercial buildings and the like, lies in the range of 22° C. to 23° C., and relative humidity typically is in the range of 50% to 65%. This relative humidity corresponds to a range between 9.73 g and 13.4 g water per kilogram air (absolute humidity).
When outside conditions are hot and moist (for example, 33° C. and 90% relative humidity), humidity ratio is around 35.4 g of water per kilogram of air. If this air is transferred to a location and cooled to a lower specified temperature range by removal of sensible heat, the relative humidity may reach 100% as the air cools and condensation then occurs with further cooling. In this example, up to 20 g of water per kilogram of air must be removed in order to produce a specified relative humidity of 55% at 23° C., as the external air is cooled to the specified internal temperature.
There are various known techniques for dehumidifying air within a process or location. One method is by thermal condensation which involves drawing air across a cold surface. As the air cools and relative humidity reaches 100%, water vapour in the air condenses as liquid water droplets. For example, a fan may draw moist air over a condensing coil maintained at or less than 11° C. The condensing coil causes condensation of some of the water vapour in the air, leaving air at a maximum humidity ratio of 9.9 g/kg air. The air is then re-heated to a temperature suitable for supply to the conditioned space, for example to around 18° C.
However, this method requires large amounts of mechanical energy, usually converted from electrical energy. This mechanical energy is used to operate a reverse Rankine cycle heat pump that must provide refrigerant fluid cooled to temperatures at or below 11° C. in the condensing coil to achieve an absolute humidity of less than 9.9 g water per kilogram of air (corresponding to a specified relative humidity of around 50% at 22 C in the conditioned space). This method of cooling supply air results in a substantial sensible cooling load as the supply air is cooled to around 11° C., as well as the latent energy load required to condense water vapour from the supply air at this temperature. This latent energy component varies with ambient humidity. For example, removal of 20 g of water vapour requires removal of around 45,000 J of latent energy. This is a typical peak demand for 1000 m2 of office space, representing a continuous latent cooling load of up to 45 kW per thousand square meters of ventilated space. The supply air temperature is reduced to less than 12° C. and then re-heated to the specified final supply air temperature. Further sensible heat energy is required to cool and then reheat the air.
Reverse Rankine cycle heat pump air conditioning equipment of this type must be capable of handling the latent load imposed by high humidity supply air passing over a condensing coil, which is chilled to around less than 11° C. to produce the required final absolute humidity in the supply air. When operating in conditions of high ambient temperature and high humidity, both the sensible and latent cooling loads are large and these loads must be met by typically a reverse cycle heat pump. These pumps work less efficiently as the temperature differential increases in hot humid periods, requiring increased input of electrical or mechanical energy, usually sourced from a fossil or nuclear fuel powered generator. The plant must be sized to operate at the peak cooling load required by the supply air system but typically operates at a lower power resulting in further inefficiency of conversion of input energy during operation at these times. In urban areas building air conditioning can form up to 35% of the total load on the electrical grid, at peak times in summer months. This load contributes significantly to the required grid capacity and therefore the cost of the electrical grid, which may result in oversizing of electrical generating plants to meet this peak summer cooling load, and consumes large amounts of fossil fuels, producing very significant amounts of green house gases.
Another known dehumidification method uses ionic membranes. Apparatus employing ionic membrane technology operate at a molecular level. Water vapour is removed through electrolysis. This dehumidification occurs at rates too low for practical use in air conditioning systems.
Yet another method involves the use of adsorption/desiccant technology. Dehumidification apparatus utilizing this technology works by exposing the high relative humidity supply air to a desiccant, which adsorbs moisture when water molecules bind to the desiccant surface. When saturated, or nearly saturated, the desiccant is removed from the moist supply air path and the adsorbed moisture is removed from the desiccant, through the application of heat.
The only commercially available adsorption systems known to the Applicant that directly remove water from the air, utilise a large wheel containing an extensive honeycomb mesh coated with desiccant material that is rotated through two separated air streams. The first air stream is a stream of hot air to dry the desiccant material, and the second air stream is the supply air stream to remove water vapour from supply air. Water vapour is removed directly from the air stream by adsorption and then as the wheel rotates, the saturated desiccant is returned to the regenerating stream of hot air that vaporizes adsorbed water from the honeycomb mesh during the regeneration cycle. The desiccator wheel thus rotates through alternating adsorption and regeneration sections. The high temperature air required for regeneration of the desiccant is either provided by direct heating from a dedicated source (typically provided by burning fossil fuel), or by indirectly produced thermal energy supplied to large heat exchangers positioned in the air stream (the large size is dictated by the low specific heat capacity of air). The rotating wheel is expensive and prone to damage over time due to its large size, and delicate honeycomb structure. Seals are required to prevent admixing of the two streams of air, and these seals are also prone to mechanical failure over time. The regeneration cycle is performed at high temperatures, close to the boiling point of water requiring substantial thermal energy input, typically from a fossil fuel source such as a gas burner. The energy supplied must exceed the latent load of the supply air stream, to regenerate the desiccant material. The desiccant material on the honeycomb is at a high temperature at the conclusion of the regeneration cycle and on rotation into the supply air stream, adds temperature to the supply air resulting in an increased sensible cooling load which must be met elsewhere in the supply air conditioning process.
It is an object of the present invention to overcome at least some of the aforementioned problems or to provide the public with a useful alternative.
Any discussion of documents, acts, materials, devices, articles or the like, which has been included in the present specification is solely for the purpose of providing a context for the present invention. It should not be taken as an admission that any or all of the previous discussion forms part of the prior art base or was common general knowledge in the field of the invention as it existed before the priority date of any of the claims herein.